Communication systems typically include a plurality of communication devices, such as mobile or portable radio units, dispatch consoles and base stations (sometimes called base site repeaters) that are geographically distributed among various base sites and console sites. The radio units wirelessly communicate with the base stations and each other using radio frequency (RF) communication resources, and are often logically divided into various subgroups or talkgroups. Communication systems are often organized as trunked systems, where the RF communication resources are allocated on a call-by-call basis among multiple users or groups. Wide-area trunked systems are sometimes organized into a plurality of “zones,” wherein each zone includes multiple sites and a central controller or server (“zone controller”) for allocating communication resources among the multiple sites.
Next generation communication systems have begun to use Internet Protocol (IP) multicasting techniques to transport packet data representative of voice, video, data or control traffic between endpoints (or “hosts” in IP terminology). In such systems, host devices, including base stations, consoles, zone controllers, and in some instances, wireless mobile or portable radio units in different zones that desire to receive packets for a particular call, send Internet Group Management Protocol (IGMP) Join messages to their attached routers, causing the routers of the network to create a spanning tree of router interfaces for distributing packets for the call. Presently, there are two fundamental types of IP multicast routing protocols, commonly referred to as sparse mode and dense mode. Generally, in sparse mode, the spanning tree of router interfaces is pre-configured to branch only to endpoints having joined the multicast address; whereas dense mode employs a “flood-and-prune” operation whereby the spanning tree initially branches to all endpoints of the network and then is scaled back (or pruned) to eliminate unnecessary paths.
A problem that arises in IP multicast communication systems, most particularly in very large systems comprising hundreds of sites and/or zones, is that the multicast spanning tree is so large that the time and/or the amount of traffic generated by the selected multicast routing protocol to establish the spanning tree may adversely affect call set-up times or voice quality as each site competes for limited site bandwidth. It would be desirable to provide an apparatus and method for routing IP multicast packets in a multi-zone system in a manner that does not require multicast spanning trees to span multiple zones. Advantageously, the apparatus and method will provide for a plurality of multicast spanning trees each constrained within a single zone, thereby allowing the spanning trees to converge much more quickly than would be possible across multiple zones. The present invention is directed to satisfying these needs.